The present invention relates to sampling apparatus for use in gas chromatographic instruments operating in accordance with the head space method and particularly relates to a sampling apparatus and system for feeding a carrier gas into a sample vessel closed by a self-sealing diaphragm, removing a gas sample from the sample vessel, and purging the gas line of the sampling device without the necessity to supply carrier gas continuously through the apparatus.
Gas chromatographic vapor space analyses (head space) is a known technique particularly useful for the determination of volatile components in samples of heterogeneous composition where usual syringe injection fails. It is also useful for the analysis of liquids containing a high proportion of non-volatiles. In practicing the head space method, it will be appreciated that sample vessels closed by self-sealing diaphragms are used in connection with the instrument analyzing the sample. As known, a state of equilibrium obtains in the head space of the vessel above the sample liquid. In that state, the partial pressures of the sample components in the head space are a direct function of the composition of the sample. In a sampling device operating in accordance with the head space method, the sample is taken from this head space and supplied to the separating column of a gas chromatograph. To that end, prior sampling devices have included a hollow needle which is pushed through the self-sealing diaphragm and into the head space above the liquid in the sample vessel. The hollow needle lies in communication with the inlet of the injection block of the gas chromatograph. The injection block also communicates with a carrier gas conduit having a shut-off valve. With the shut-off valve initially open and the needle extending through the diaphragm of the sample vessel into the head space, carrier gas enters the head space whereby pressure builds up within the sample vessel. The partial pressures of the sample components in the head space, however, remain substantially unchanged. When the shut-off valve is subsequently closed, the carrier gas pressure at the injection block is reduced and pressure compensation causes a sample of the gas from the head space to flow through the needle into the injection block. After a predetermined time, the shut-off valve is again opened, whereby sample feeding is terminated. The gas sample, which has reached this injection block, however, is transported to the separating column of the gas chromatograph by the carrier gas flow.
To insure that carrier gas will not continue to flow unrestrictedly out of the needle after the needle has been withdrawn from the sample vessel, the needle has been disposed in a piston which is sealingly movable relative to a cylinder. This cylinder has a restricted outlet along its side and is closed by a self-sealing diaphragm at its end face in registry with the sample vessel. A compression spring acts between the piston and cylinder and biases the piston for movement in a direction away from the self-sealing diaphragm and in a manner to retract the hollow needle into the interior of the cylinder.
In this prior art arrangement, the hollow needle is stationary and communicates continuously with the injection block and the carrier gas conduit. In use, the cylinder is guided for longitudinal movement relative to the stationary hollow needle. The sample vessel is placed with its self-sealing diaphragm in engagement with the end face of the cylinder and is shifted upwardly, whereby the cylinder is pushed back about the piston and the hollow needle enters the sample vessel through the self-sealing diaphragm. In the rest position of this apparatus, a flushing or purging stream of gas flows continuously through the hollow needle, the intensity of the stream being determined by the restricted outlet from the cylinder. This purging stream insures that no vapors are carried in the hollow needle from one sample to the next sample. The foregoing arrangement is disclosed in U.S. Pat. No. 3,545,279 issued Dec. 8, 1970 and which Patent is of common assignee herewith.